In a recent reply to the post "Filters in Australia" mention was made of Xume filter holders. I have not come across these, and would find them very useful I think. However, I cannot seem to get an answer from the Manfrotto web site: can they be used on the front of an enlarging lens with a diameter of approx. 35mm, with no filter thread. I have just been give some old enlarger lenses (e.g. Rodenstock Trinar) and would like to be able to fit easily a Baader U filter to try them for UVR photography
Thank you.

They come in two forms, called filter adapter and lens adapter.
One to be attached to the lens with a male thread, just like any threaded filter.
One to be attached on the filter with a female thread.

The two types are connected together magnetically at their ring-shaped flat surfaces-
There is a protruding edge to keep the two parts mating concentrically.

There are some demonstration videos on the net showing this.

For more or less temporary tests I would use a step ring and some grey-tack or similar putty.
Get a ring that fit the Baader U with the second diameter reasonably close to the dimension of the lens to make it easy to use the putty.

Yes, the Vid-Atlantic website went down after I looked it up and got the link. However, the Ebay link which Mark posted shows Vid-Atlantic as the seller there. That linked item is the one which I have. The diameter of the screw ends is fairly wide so I should note for the record that this filter holder cannot be clamped onto a too narrow outer portion of the lens.

There is also a do-it-yourself method for temporarily fitting a filter ring. You wrap the outer edge of the lens with a rubber band or narrow rubber gasket or narrow strip of foam paper or any other similar tight-fitting spongy material. Then you "force" a step ring over the compressible band. The band "bounces back" and holds the step ring in place. Now attach the filter to the step ring. The band & step-ring can be dislodged if bumped, but it works pretty well for some lenses. Key to this construction is having an lens, band and step-ring of appropriate diameters so that everything will be fittable.

I hope I've described that so it makes sense.

There is a filter holder made by Lee which makes use of this idea. A "sleeve" fits over the lens and somehow holds the filter.

An extra step ring could remedy the problem. For example, with the clamp adapter I have, I typically use the 60f to 52m step-down ring that is part of the set. I then attach a 55m to 52f step-UP ring. Now the filter can be attached in the usual manner. (Most of my filters are 52mm for use in UV work.)

***

I have a box full of step rings accumulated over the years. But of course I never have the size I need when I rummage through it.

ADDED LATER: I said that I "typically" use the 60f to 52m step-ring with my clamp adapter. But I should add that I rarely use the clamp adapter. For one thing, I have been lazy about acquiring nylon screws to prevent scratching the lens barrel. And also some of the small threadless enlarger lenses useable for UV are too small for this clamp adapter. I've fallen into the habit of simply hand-holding filters over such lenses. This is not a method I can recommend - especially for rigorous documentary or studio shots. <smile>